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maybe they just like using a specific radius for the corners of their products

On a more historical note: http://www.landsnail.com/apple/local/design/design2.html describes a design system for early Apple products that specified corner radii for various situations.

That is a great site, thanks for the link!

It could have the same radius and not fit. It has the same radius and the same width.

Or perhaps there's a relationship between a border length and its radius* - some sort of eye-pleasing ratio?

In which case, only the width would have to be the same.

*Someone more interested than me could perhaps see if any relationship exists.